Orthogonal frequency division multiplexing, or OFDM,
is a popular digital transmission method.
OFDM transmit signals over multiple frequency bands
and is used widely in many common wireless communication
standards such as Wi-Fi, LTE, and 5G.
In this video, we will look at OFDM and how
OFDM signals are generated.
We will also look at how the signal is
processed at the receiver.
We will discuss the importance of OFDM modern wireless
standards.
And finally, we will show how to use MATLAB, Simulink,
and wireless communication products
to build, test, and analyze OFDM platforms as part
of your wireless communications system design.
OFDM is a spectrally efficient transmission method
which allows you to send a large amount of data
within a given bandwidth.
In OFDM, data streams are mapped to signal components called
sub-carriers, which are overlapping equally
spaced complex sinusoids.
The sub-carriers are orthogonal to one another,
so that at the center of each sub-carrier
the signal contribution from the other carriers is 0.
OFDM wave form generation at the transmitter
involves several key components.
The information comes from higher layers as a bit stream.
It is then encoded and modulated into symbols using a modulation
mapper such as QPSK or QAM.
The IQ samples are then aligned in a two-dimensional resource
grid.
The y-dimension represents the frequency domain,
while the x dimension represents the time.
Next, an inverse fast Fourier transform or IFFT
is applied to the resource grid to create the OFDM symbol.
A cyclic prefix is then added to the front of every OFDM symbol.
The cyclic prefix takes the last group
of samples in the OFDM symbol, copies,
and moves them to the front.
Adding a cyclic prefix to each OFDM symbol,
greatly simplifies equalization operations at the receiver.
Pilot samples, also known as reference signals,
are important element in OFDM demodulation.
Pilot symbols are one element of a resource group,
but there are other elements as well.
In the following resource grid, the reference signals
are represented by the white rectangles.
These reference signals are known
data that are inserted periodically
in time and frequency to help with the channel estimation
and equalization later in the receiver.
At the receiver, the reference signals
are first recovered then used to estimate the channel.
This channel can then be used to equalize the received symbols.
In OFDM, equalization reduces to a complex scalar
multiplication.
After equalization, we obtain a modulated symbols
which are then demodulated and decoded to yield
our original data raw bits.
There are many reasons why OFDM is one of the most widely used
digital transmission schemes.
OFDM is very robust at handling signals
in environments with lots of scattering and multipath
fading.
It's also a flexible scheme allowing you to easily create
signals of arbitrary bandwidth by adding or removing
some carriers.
Lastly, OFDM fits well with MIMO and massive MIMO systems
because the presence of multiple antennas
allow for better exploitation of scattering environments
and more efficient use of OFDM data streams.
MATLAB and its wireless communications products
such as 5G, LTE, and wireless LAN toolbox
provide functions tools and apps to work directly
with OFDM signals.
You can generate standard compliant OFDM wave forms
and perform end-to-end simulations of 5G, LTE, and W
LAN systems.
You can analyze the received OFDM signals
with metrics such as EVM, ACLR, bit error rate, block error
rate, and throughput which will help
you verify the compliance of your wireless system
to the standard specifications.
You can also connect your OFDM signals directly
to RF instruments for over-the-air testing using
the wireless waveform generation app.
You can implement OFDM receiver operations
such as synchronization, channel estimation, equalization,
and demodulation using built-in functions
for wireless communications toolboxes.
You can use MATLAB tools to visualize your data
at every step.
You can use wireless HDL tool box and HDL coder to implement
OFDM algorithms onto FPGA's and ASIC's.
Using HDL Verifier, you can then validate
that your low-level implementation matches
your golden reference design.
For more information, explore the wireless communications
solutions page as well as the OFDM documentation.